Abstract
Bacillus subtilis biofilms have a fundamental role in shaping the soil ecosystem. During this process, they unavoidably interact with neighbour bacterial species. We studied the interspecies interactions between biofilms of the soil-residing bacteria B. subtilis and related Bacillus species. We found that proximity between the biofilms triggered recruitment of motile B. subtilis cells, which engulfed the competing Bacillus simplex colony. Upon interaction, B. subtilis secreted surfactin and cannibalism toxins, at concentrations that were inert to B. subtilis itself, which eliminated the B. simplex colony, as well as colonies of Bacillus toyonensis. Surfactin toxicity was correlated with the presence of short carbon-tail length isomers, and synergistic with the cannibalism toxins. Importantly, during biofilm development and interspecies interactions a subpopulation in B. subtilis biofilm lost its native plasmid, leading to increased virulence against the competing Bacillus species. Overall, these findings indicate that genetic programs and traits that have little effect on biofilm development when each species is grown in isolation have a dramatic impact when different bacterial species interact.
Highlights
Soil bacteria have a central role in shaping soil ecology
The soil microorganismic community is composed of diverse populations of bacterial species that dramatically affect the availability of soil nutrients and plant diversity.[1,2,3,4]
We found that within 3 days, B. toyonensis colony is mostly eradicated by B. subtilis (Figure 1n–p)
Summary
Soil bacteria have a central role in shaping soil ecology. The soil microorganismic community is composed of diverse populations of bacterial species that dramatically affect the availability of soil nutrients and plant diversity.[1,2,3,4] The immense bacterial diversity within the soil leads to unavoidable interspecies interactions, which form a structured microbial community.[5,6]Generation of antagonistic and mutualistic behaviours, mediated by exchange of small diffusible secondary metabolites, enables bacterial adaptation to the complex communal life.[6,7] Such communication can induce resistance to various antibiotics, or can eliminate rival bacterial species competing for limited nutrients. Biofilms are multicellular communities of surface-associated bacteria enveloped in a self-produced extracellular matrix.[8] The extracellular matrix isolates the bacteria in the biofilm from the external environment and protects them from antibiotics, sterilising agents and the immune system.[9] Here we studied the interactions between two robust biofilm formers—Bacillus subtilis and the closely related Bacillus simplex. Both bacterial species reside in and compete for the same ecological niche—the soil.[10,11]
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